Method and apparatus for the production of liquid gas products

Abstract

 A method and apparatus are disclosed for increasing the production of liquid natural gas and conserving energy and reducing vapor production, while reducing equipment cost, in a liquid natural gas manufacturing installation. A liquid gas stream from a main exchanger is fed to a bi-phase rotary separator, which separates a vapor phase from the liquid phase without requiring an expansion valve and flash device, or liquid pump, as is commonly used. Optionally, work can also be extracted from the stream by the separator.

Description

[0001] The present invention relates to a method and apparatus for the production of liquid gas products, such as liquid natural gas, and, more particularly, relates to a reduction in the quantity of formed flash gas and in equipment requirements in a liquid gas manufacturing installation through the novel utilization of a bi-phase rotary separator for separating liquid and gas phases which, optionally, can also extract work from a liquid gas flow.

[0002] In liquid natural gas manufacturing facilities wherein liquid natural gas is conveyed from the manufacturing facility to a storage location subsequent to being flashed in a low pressure flash, a continuous stream of the liquid natural gas is conducted, as is well known, from a main exchanger of the liquid natural gas manufacturing facility through either a Joule-Thomson valve, or a reversely operating pump which removes work from the flow, and then to a low-pressure flash in which the effluent from the Joule-Thomson valve or a reversely operating pump is divided into a first flow consisting of liquid natural gas, and a second flow consisting of a natural gas vapor. The liquid natural gas is conducted through the intermediary of a suitable transfer pump to a storage facility, and the natural gas vapor is employed as fuel within the operating facility or plant.

[0003] Basically, the Joule-Thomson valve or reversely operating pump operate on the liquid natural gas stream flowing from the main exchanger of the liquid natural gas manufacturing facility at a very low, i.e., supercooled, temperature, thereby extensively reducing the temperature and pressure of the liquid flow. When a reversely operating pump is used within the facility, work is extracted and converted into mechanical or electrical energy through suitable shaft-coupled compressors, pumps or generators in order to power other installations or plant components. Thus, the energy state change through the Joule-Thomson valve, or the work recovered by the reversely operating pump, although relatively small in quantity, significantly enhances the production of liquid natural gas when flashed, while producing a lower volume of flash gas or natural gas vapor, thereby improving the economical operation of the manufacturing facility.

[0004] Although the utilization of Joule-Thomson valves and reversely operating pumps which extract work, such as centrifugal pumps or the like, result in an energy state change or the extraction of work from liquid streams under pressure, such as a cryogenic processing system for liquid natural gas which is conducted under high pressures and extremely low temperatures from the main exchanger of a liquid natural gas manufacturing facility, the energy state change or the work extracted has, generally, not been adequate to provide a degree of reduction in flash gas or natural gas vapor subsequent to flashing in a low pressure flash to a level of flash gas which meets the gas fuel requirements within the facility. Instead, an appreciable excess or waste of natural gases occurs, with a concommitant reduction in the production of processed liquid natural gas, so that the economic production potential of the liquid natural gas manufacturing facility is not fully realized.

[0005] In order to improve the economics of liquid natural gas production and to reduce the quantity of flash gas and resultingly increase the yield of liquid natural gas, an arrangement and method is disclosed in U. S. Patent No. 4,456,459 for extracting work from a flow of liquid natural gas at extremely low temperatures with a hydraulic expander. The effluent from the hydraulic expander, when flashed in a low-pressure flash, produces a higher yield of liquid natural gas and, consequently, a lower proportion of flash gas, with an additional conservation of energy. The hydraulic expander is used in lieu of the conventional Joule-Thomson valve or reversely operating pump and the work extracted therefrom may be usefully employed in the facility to operate various power-driven components through suitable shaft-coupled compressors, pumps or generators. In a particularly advantageous arrangement, a Joule-Thomson valve is interposed in a flow conduit in parallel bypass relationship with the conduit incorporating the hydraulic expander and, in essence, is positioned intermediate a main exchanger of the liquid natural gas production facility and the low-pressure flash. The Joule-Thomson valve is closed during normal operation of the facility so as to render it inoperative and the entire flow of liquid natural gas passes through the hydraulic expander. The Joule-Thomson valve is rendered operative in an opened flow-through condition during periods when the hydraulic expander is shut down, or inoperative, to facilitate the continuous and uninterrupted operation of the liquid natural gas production facility, albeit at a somewhat lower degree of efficiency, without necessitating any shutdown of the system.

[0006] Although the just-described arrangement using a turbo-expander is advantageous in reducing the amount of produced vapor while extracting work from a flow of liquid natural gas, the vapor and liquid phase output from this device, in the form of effluent, must still pass to a conventional low pressure flash device for separating the liquid and vapor phases, with the liquid phase from the flash device then being pumped in conventional manner to a storage facility. The provision of a flash device and liquid pump naturally adds to the capital and maintenance costs of a liquid natural gas manufacturing installation.

[0007] There is still a need for an improved method and apparatus for separating the liquid and vapor phases of a very low temperature, i.e., supercooled, liquified gas stream, such as liquid natural gas, which eliminates the requirement for a flash device and a liquified gas pump for pumping the liquid phase to a storage facility.

[0008] A need also exists for a method and apparatus for separating the liquid and vapor phases of a low temperature, i.e., supercooled, liquid gas stream which exhibits an improved separation efficiency for liquid over separation systems employing a Joule-Thomson valve and downstream flash device.

[0009] It would also be beneficial if there was an improved method and apparatus for separating the liquid and vapor phases of a liquified gas stream using a bi-phase rotary separator, wherein work is optionally extracted by the separator.

[0010] Accordingly, the present invention provides an apparatus for producing liquified gas with a reduced amount of gas vapor in a liquid gas manufacturing facility, comprising: a first flow conduit for receiving a stream of liquid gas from a liquid gas manufacturing facility; a bi-phase rotary separator having an inlet connected to said first flow conduit, a vapor outlet, and a liquid outlet; means for coupling the liquid outlet of the bi-phase rotary separator to a liquid gas storage means; and means for conducting gas vapor from the vapor outlet.

[0011] In another embodiment, the present invention provides a method of producing liquified gas using the above apparatus.

[0012] Since the bi-phase rotary separator is also capable of extracting work from the incoming liquified stream, it is possible, with some loss in conversion efficiency, to also use this extracted work to operate other devices, e.g., compressors, pumps, etc., associated with a liquid natural gas production facility.

[0013] It is also possible to use the bi-phase rotary separator in a parallel bypass relationship with a conventional Joule-Thomson valve or turbo-expander, so that if the bi-phase rotary separator is taken out of service, an alternate operational feed stream processing flow path still exists, thereby preventing system shutdown.

[0014] The single figure illustrates a schematic flow diagram of a stream of liquid natural gas conveyed from the main exchanger of a typical liquid natural gas manufacturing facility through the bi-phase rotary separator.

[0015] The figure shows arrangement 10 for producing liquid natural gas. The invention can be used with other liquid gas stream as well.

[0016] Feed conduit 12 is from a main exchanger (not shown) of a typical liquid natural gas manufacturing facility, which is well known in the technology.

[0017] Conduit 12 connects to a bi-phase rotary separator 14. The bi-phase rotary separator may be a commercially available bi-phase rotary separator, such as manufactured by Biphase Energy Systems of Santa Monica, California. Bi-phase rotary separators are, per se, known. A description of such devices can be found in U. S. Patent Nos. 3,879,949; 4,298,311; and 4,336,693.

[0018] The bi-phase rotary separator 14 extracts work from the fluid flowing through it. It produces an effluent which is predominantly liquid natural gas and which is then conveyed into the conduit 16. Typically, the system operates at extremely low temperatures, such as supercooled temperatures, the liquid natural gas in the conduit 12 being, for example, at a temperature of -143°C (-225°F) and at a pressure of 3,689 kPa (535 psia) upstream of the bi-phase rotary separator, while subsequent to passing through the bi-phase separator, in the conduit 16, the temperature of the effluent may be found to be at -158°C (-251.7°F) and at a pressure of 1,862 kPa (270 psia). The liquid effluent in conduit 16 is conducted directly into a liquid natural gas storage tank 27, without the need for a pump to increase the liquid pressure for transport to the tank.

[0019] The vapor in the conduit 18 exiting from the bi-phase rotary separator passes through conduit 20 and may be utilized as fuel for driving other components within the facility, or may simply be torched if it is not needed for any purpose.

[0020] The bi-phase rotary separator 14 may be shaft-coupled to suitable compressors, pumps or generators, enabling the work extracted from the fluid flowing through line 12 to be converted into usable mechanical and/or electrical energy, thereby resulting in a considerable energy saving to the overall system. However, to maintain a high efficiency in separating the liquid phase, there will typically be no output work drawn from the separator.

[0021] Conduit 26 connects into the conduit 12 in a parallel bypass flow relationship with respect to separator 14. Interposed in conduit 26 is a Joule-Thomson valve 28. The outlet of the Joule-Thomson valve feeds a conventional low pressure flash device 30, typically a large drum, which has vapor and liquid outlets feeding into conduits 20, 32, respectively. The liquid outlet is connected to a pump 24 for pumping the liquid natural gas to storage facility 27 via conduit 22. During normal operation of separator 14, the Joule-Thomson valve 28 is closed the entire flow of liquid natural gas from the manufacturing facility to flow through the bi-phase rotary separator 14. If, for any reason, the rotary separator 14 is taken out of service, valve 31 is closed and Joule-Thomson valve 28 is opened. Liquid natural gas expands across valve 28 and enters flash device 30, where it is separated into liquid and gas phases.

[0022] Table I shows operation of the arrangement 10 employing the bi-phase rotary separator 14 for a typical flow of liquid natural gas fed from a liquid natural gas manufacturing facility.

[0023] The feed stream conducted into conduit 12 from the main exchanger of the liquid natural gas (LNG) manufacturing facility is separated into vapor, removed via line 20, and liquid natural gas conveyed via line 22 to storage. As shown, of 100 moles initially fed, 90.71 moles are obtained as liquid natural gas which is passed to the liquid natural gas storage, whereas 9.29 moles are present as vapor.

[0024] Table II illustrates the production of liquid natural gas via Joule-Thomson expansion.

[0025] Using Joule-Thomson expansion, the vapor from the low-pressure flash device 30 consists of 9.76 moles, and the liquid natural gas conducted to the storage through conduit 22 consists of 90.24 moles for each 100 moles of fed liquid natural gas from the manufacturing facility.

[0026] Use of bi-phase rotary separator 14 instead of Joule-Thomson expansion gives 90.71/90.24 = 1.005 times more liquid natural gas production. Vapor production using the bi-phase rotary separator is (9.76 - 9.29)/9.76 x 100% = 4.816% lower than that obtained with the Joule-Thomson valve.

[0027] Viewed another way, for a main exchanger feed of 12.2 Kg/sec at 3,869 kPa (535 psia) and -143°C (-225°F), the bi-phase rotary separator, extracting no work, produces a vapor stream of 1 Kg/sec at (124 kPa (18 psia) at -158°C (-252°F) and a liquid stream of 11.2 Kg/sec at 1,862 kPa (270 psia) and -158°C (-251.7°F). By contrast, the Joule-Thomson valve arrangement employing a flash device 30 and liquid stream pump 24 produces a liquid stream of 10.948 Kg/sec and a vapor outlet of 1.252 Kg/sec, with the vapor temperature and pressure being -158°C (-251.9°F) and 124 kPa (18 psia), respectively, and the liquid temperature and pressure being -158°C (-251.6°F) and 758 kPa (110 psia), respectively.

[0028] Use of a bi-phase rotary separator 14 instead of the Joule-Thomson valve 28, flash device 30 and liquid pump 24, there is obtained a higher yield of liquid natural gas with a concurrent reduction in vapor, and with usable energy being available through work optionally recoverable by means of the bi-phase rotary separator. In addition, capital and maintenance costs associated with a flash device 30 and pump 24 can be reduced, thereby rendering the entire liquid natural gas production system more economical in operation.

[0029] The use of the Joule-Thomson valve flash device amd pump in the system in a flow bypass relationship with the bi-phase rotary separator ensures that during periods when the rotary separator is inoperative, such as during repairs or replacements, the arrangement may continue operation at a reduced efficiency in the output or yield of liquid natural gas. It is also possible to use a turbo-expander in a bypass relationship to the bi-phase rotary separator 14.

Claims

1. An apparatus for producing liquified gas with a reduced amount of gas vapor in a liquid gas manufacturing facility, comprising:

a first flow conduit for receiving a stream of liquid gas from a liquid gas manufacturing facility;

a bi-phase rotary separator having an inlet connected to said first flow conduit, a vapor outlet, and a liquid outlet;

means for coupling the liquid outlet of the bi-phase rotary separator to a liquid gas storage means; and

means for conducting gas vapor from the vapor outlet.

2. Apparatus of Claim 1, wherein the gas is a natural gas.

3. Apparatus of Claim 1, wherein the bi-phase rotary separator includes means for extracting work from the stream of liquid gas.

4. Apparatus of any claims 1 to 3, further comprising a Joule-Thomson valve for expanding the stream of liquid selectively connected in parallel to the bi-phase rotary separator.

5. Apparatus of any of Claims 1 to 3, further comprising a turbo-expander for expanding said stream of liquid connected in parallel to the bi-phase rotary separator.

6. A method for producing liquified gas with a reduced amount of gas vapor in a liquid gas manufacturing facility, comprising the steps of:

supplying a stream of liquid gas from a liquid gas manufacturing facility into a bi_-phase rotary.separator, and separating therein the stream into liquid and vapor phases, and supplying liquid and vapor phases to liquid and vapor outlets; and

charging the liquid phase to a storage facility without further pressurizing said liquid phase.

7. Method of Claim 6 wherein the gas is a natural gas.

Drawing